SE467096B - PROCEDURE AND DEVICE FOR THE PREPARATION OF A GLASSED CERAMIC TILE PLATE AND TILE PLATE RECOVERED THEREOF - Google Patents

Description

2 Such bodies undergo chemical and structural changes during a heat treatment and acquire essentially new mechanical properties, ranging from low toughness in the raw state to a hard, tough and brittle state after the heat treatment.

Accordingly, in the present specification, the term "burning of ceramic bodies" generally refers to this heat treatment which causes the above-mentioned significant change of the mechanical characteristics of ceramic bodies by means of chemical reactions, crystal modifications and component melting.

Biscuit is then glazed and further fired to bring about in the glaze the structural change - which is strictly dependent on the nature of the components of the glaze - which for simplicity will hereinafter be referred to as vitrification.

This expression may in some cases not be completely correct for all glazes used in the ceramic tile manufacturing industry and which during firing can melt almost completely or only to a small degree, in the latter case the phenomenon is almost similar to sintering.

However, since the firing of glaze on ceramic materials is well known in the art, such a linguistic simplification cannot lead to any misunderstanding. By glaze is meant here any composition which is adapted to develop a glassy, transparent or opaque surface during heating, which can be glossy or matte, either in a completely fried form, ie. melted beforehand, or raw form or in the form of various compound mixtures.

This double firing process, first of the ceramic body and then of the glaze, has advantages, but also disadvantages. A first problem is that the first firing, the subsequent cooling and then the second firing for glazing the glaze constitute a process which consumes a lot of heat energy and a long time and which requires apparatus volumes which are large in relation to the volume produced. tile or tiles.

One might think that in the case of double firing, the first firing to produce the biscuit, if carried out in a professional manner, can give a biscuit with optimal characteristics as a result solely of the nature of the starting clay; the fact is, however, that the requirements arising from the subsequent glazing in practice determine the production of the biscuit.

The glazes are generally prepared by wet grinding so that they e.g. contains 30 - 60% water.

If dry glazes are used, a wet treatment is then required to ensure adhesion to the ceramic body.

When the glaze is applied to biscuits with a porous structure, ie. which is not glazed, the high presence of water is not a source of difficulty; under such conditions, the biscuit easily absorbs the water and the glaze therefore adheres well to the body.

However, since the biscuit is vitrified, special treatments are required before the actual firing of the glaze, especially a preheating to obtain rapid evaporation of the water; nevertheless, if one wishes to obtain an acceptable adhesion of the glaze, such treatments prove to be extremely critical and thus entail considerable difficulties with regard to the procedure in question.

Consequently, the problems which arise in the glazing of vitrified ceramic bodies nullify the apparent freedom to form the ceramic body independently in double firing, since such independent formation will only allow the formation of tiles with serious complications at the expense of serious complications. vitrified ceramic bodies with desired mechanical strengths 467 096.

Due to the problems encountered in the double firing process, it has recently been proposed to burn both the clay mixture and the glaze in a single process step - the so-called the single burn procedure. According to this procedure, the raw ceramic bodies are dried, preheated and glazed when they are sufficiently hot to enable the water contained in the glaze to be evaporated. However, the preheating does not prevent a certain amount of the water from being absorbed by the raw clay mixture - and also in an uneven way so that in practice the absorption turns out to be greater at the edges of the object as edges cool faster.

This phenomenon can lead to cracking of the article at its edges, thus reducing its strength.

Another inherent difficulty with the single firing process is that it requires somewhat complicated movements of the raw plates, which must be glazed in this condition, and whose delicate nature is known.

In the single firing process, the raw glazed plate is then fired in a single cycle up to a temperature which burns both the ceramic mixture and the glaze (glazing of the glaze).

As this requires only one oven and only one firing step, the single firing is clearly a significant improvement over the traditional double firing: the apparatus becomes cheaper and the heat energy, which is lost between the firing of the biscuit and the firing of the glaze, is saved.

If the reactions which occur during firing are sufficiently known, in the case of single firing it is also possible to obtain almost all the effects which are typical in double firing, although it may appear to be a limitation to have to provide a firing temperature for the clay mixture, which is compatible with the characteristics of the glaze, which is glazed during the same firing, with the consequent loss of independence in the design typical of double firing.

However, the greatest difficulty in single firing is the tendency of the gases released during the firing of the ceramic body to spoil the smoothness and compact nature of the glaze.

The significance of this phenomenon is easy to remember if a tile weighing 1 kg, formed by a mixture containing 10% calcium carbonate and brought to a temperature of 1000 ° C, releases a carbon dioxide volume of about 90 liters.

The gas released from the ceramic body passes through the glaze, leaving small bubbles, which to some extent leaves it in a porous state. Thus, although single firing eliminates the adhesion problems of glazed ceramic bodies in double firing in the case of vitrified ceramic bodies, single firing leads to glazes which, although having excellent adhesion, have a relatively porous consistency.

It has also been proposed to apply an insulating layer, called an engobe, between the glaze and the ceramic body in the single firing process to reduce the possibility of interaction therebetween during firing. However, the application of the engoben reduces the satisfactory adhesion of the glaze, which is characteristic of single firing.

In summary, double firing leads to tiles of a porous ceramic body, which has lower mechanical strength, but an adhesive and extremely compact glaze, while single firing gives rise to plates with a glazed ceramic body with high mechanical strength and with a glaze which adheres satisfactorily but is relatively porous. The object of the present invention is to overcome the difficulties in the above-mentioned methods of firing tiles and tiles by means of an improved method for glazing and firing such tiles and also an apparatus for performing such a method.

The invention thus relates to a method for producing a glazed ceramic tile or tile in two layers consisting of ceramic material resp. glaze, without any intermediate layer, wherein. starting from a raw, dried, ceramic body, which is placed in an oven for a heat treatment in two stages with intermediate interruptions in the heat treatment for applying the glaze, which is characterized in that the plate during the first heat treatment step is moved on a transport means by an oven zone with a sufficiently high temperature and for a sufficiently long time for the firing reactions to be substantially completed, after which the heat treatment is stopped and the ceramic body, which is at substantially the final temperature for the first heat treatment step and is in a white annealed plastic state, is spread out a dry loose glaze, at a temperature for the glaze and within a zone with a temperature lower than the melting temperature of all components included in the glaze, the plate, when leaving the first stage, exhibiting a higher temperature than the glazing temperature of the glaze and wherein the time between the first and the second heat treatment step is so short, that the ceramic body maintains a sufficiently high temperature to remain under plastic conditions and not to become brittle, the ceramic plate being maintained or brought to a temperature sufficiently high during the first part of the second stage for the glaze to shall be vitrified by melting at least one component part therein.

The invention also relates to a tile or tile 467 096 7 obtained by this method and to a device for carrying out the method according to the invention, which comprises a kiln or chamber, in which raw, dried ceramic bodies are placed, comprising a first part in the furnace , comprising means for heating the ceramic raw bodies so that they are burned, a second part of the furnace provided with temperature control means for keeping the ceramic bodies at a regulated and decreasing temperature for continuous cooling of the bodies, means for moving the bodies in followed by the first and the second part of the furnace, and a glazing station, characterized in that the furnace is provided with a roller transport means, which forms a continuous support surface for the plates, which are spaced apart from each other and which are supported by a number of support points, for transporting the plates through the first part, where the plates are heated until the firing reactions are substantially a concluded, through the glazing zone arranged between the first and the second part of the oven, between an outlet from the first part and an inlet to the second part, and through the second part, that the glazing station comprises a spreading device which deposits dry loose glaze on the ceramic bodies and in a collecting device collects the glaze, which falls into the areas between the spaced apart plates and which is located inside an environment provided with temperature control means, which keep the temperature lower than the temperature at which the glaze undergoes vitrification and physical and chemical changes, the distance between the first part and the second part being small enough that the plates leaving the first part in a white annealed plastic state are not cooled to a brittle state during the passage therethrough.

The method will now be described in an exemplary embodiment with reference to a similarly exemplary embodiment of the burning device according to the invention, which is illustrated in the accompanying drawings, in which: Fig. 1 is a general schematic view of the device according to the invention. ; Fig. 2 illustrates in detail a part of the device of Fig. 1; and Fig. 3 shows a typical diagram of the temperature in the device of Fig. 1.

According to the invention, the ceramic bodies, which are formed in a conventional manner, are transferred in crude form, after suitable drying, to the orifice 10 in the kiln.

The kiln has a first part 11, in which the temperature increases more and more to a sufficiently high value to bring the plates therein, which are moved on roller transport means, to a firing temperature, i.e. a temperature at which the reactions for firing the crude ceramic material can be considered substantially complete.

At the end of this part 11 in the oven, the transport means moves the plates to a glaze deposition zone, which is generally denoted by 12.

This zone, which is of particular importance in the invention, is shown in Fig. 2. Reference numeral '13 denotes' the outlet for the plates from the part 11 in the oven of the conveying means, which is schematically shown as rollers 14. The roll conveying path continues approximately to zone 12, where an environment is formed in which the temperature can be regulated, for example by means of recordable openable panels 15, so that the temperature within the zone 12 remains at a preset value which is lower than that in the part 11 near its outlet mouth 13, in order to facilitate regulation of the temperature of the plates.

Within the zone 12, a device, generally designated 20, is provided for evenly spreading a dry glaze in loose form, either as barley or powder, over the plates, which are moved below transported by the rollers 14.

For simple description of its function and overall design principles, this device can be considered to consist of a container 21 for the glaze, which is fed to a discharge means 22, which spreads it evenly over the plates, which are transported below, in any number of rows, through the rollers. 14.

Below the feeding device 20 a collecting means 23 is arranged, which is intended for the glaze, which is not collected by the plates, in particular the glaze which falls into the spaces between the plates, which are transported by the rollers 14. The glaze collected in the means 23 is suitably returned to the container 21.

The schematic construction of the device 20 is given by way of example only, as it may have any known embodiment - dust type, type with vibrating net, spray or other type, requiring only a correct and continuous discharge of the glaze in the form of granules or powder. on the plates, which are transported near the device.

The plates leave the zone 12 and enter the mouth of the inlet 16 of the subsequent part 17 of the oven, where they remain at a sufficiently high temperature to finish the vitrification and curing of the glaze and are then cooled in a sufficiently slow manner to prevent build-up of impermissible internal voltages therein, whereupon they are finally taken out through the final outlet 18 in the furnace.

Construction details of the furnace are not described as the two parts 11 and 17 are very similar to a traditional furnace or kiln or a heating chamber for firing ceramic bodies to obtain the biscuit, which is formed in the double firing process. The two parts can therefore be constructed using all that is known by conventional technology in this respect.

A 67 096 10 In particular, the orifices 10, 13, 16 and 18 of the furnace will be provided with structural details, which are known per se in the field, in order to enable a regulated atmosphere to be maintained in each part of the furnace in order to favor the desired reactions during the heat treatment of the ceramic body and of the glaze.

The solution according to the present invention is based on the awareness of the possibilities of briefly interrupting the firing of the ceramic body, ie. the possibility of exposing the ceramic body to a relatively cold environment during at least one stage of firing, ie. an environment whose temperature difference with respect to the ceramic body is much larger than that inside the furnace during the firing step and the cooling step of the ceramic body.

It has in fact been found that the thermal shock which the plate undergoes as a result of the exposure to such an atmosphere is easily tolerated by the material if this in any case remains under plastic conditions and not under fragile conditions. Such plastic conditions occur when the ceramic body is in a white annealed state, ie. at a temperature of at least about 700 ° C, for the majority of the firings for ceramic bodies, which are most commonly used for the manufacture of ceramic tiles.

According to the invention, the temperature in the zone 12 is kept at a value which is such that. an overheating of the glazing distribution means 20 is avoided such that melting occurs or in any case a thickening of the glaze contained therein; this means that such bodies are not clogged and that there is no unevenness in the distribution of the glaze over the tiles.

It is also advisable to keep a relatively low temperature in the whole zone within which the glaze is distributed, in order to prevent the glaze falling on the plates from being sublimated with the following distribution in the atmosphere and contamination of the furnace and the means inside. therein.

The plates reaching the zone 12 are annealed at the temperature they had during firing in the final part of the zone 11 in the furnace: such temperature is not necessarily the maximum temperature which the plates reach in the furnace 11.

When the glazing temperature of the glaze is lower than the firing temperature, it is in fact suitable that the plates begin a cooling phase in the end zone 11 of the oven, which is such that it causes the plates to reach the glaze distribution device at the most suitable temperature, taking into account the temperature reduction. which they undergo in zone 12.

Thus, in practice, regulation of the temperature of the plates, when they receive the glaze, can be carried out by regulating the temperature at which they leave the part 11 in the oven, taking into account the lowering of the temperature which they undergo in zone 12. There is another possibility to regulate the latter temperature reduction is by influencing the temperature maintained in zone 12, provided, as already mentioned, that such a temperature does not become such that it causes physical or chemical changes. - rings in the powdered glaze, which are distributed over the white-annealed plates.

As well as the distribution of the glaze, if desired, another operation can be performed with the decoration of the plates within the zone 12, such as placing high pressure printing decks to provide decorations which require a special distribution of the dye forming the decoration. The printing decoration can be placed on the plates through automated control devices.

The glaze deposited on the plate reaches its maximum temperature and minimum viscosity exactly at the contact surface with the ceramic body, whereby the adhesion with the body is favored. 467 096 12 The fact that the residence time of the plates within the zone 12, which is required for them to receive the glaze, is short prevents them from transitioning from the white-annealed plastic state to the brittle state - at which the heat shock can produce internal stresses which form cracks in the clay material.

The spreading of the glaze as a loose powder means that the amount of glaze which adheres to the vertical sides of the plate is negligible and practically zero and thus the plate is clean at the edges, which in turn reduces the peripheral fragility of the glaze coating and the possible to its flaking.

If it is desired to apply more than one glaze, each of which requires a certain temperature of the ceramic body for its application and a subsequent temperature for curing, additional glazing stations can be provided at the part 17 of the oven, in the end part of which the plates are gradually cooled.

The features of the method according to the invention become clearer by the following description of certain embodiments thereof.

Example 1 Production of wall tiles with the following characteristics: Size: 15 x 20 cm Thickness: 7 mm Porosity: 12% Breaking strength: 200 kp / cmz Glossy glaze with a blue sand effect.

Characteristics of the mixture: Mixture of natural sand and clays in the following proportions: Majolica clay: 60% Grass clay: 25% 467 096 13 Feldspar sand: 15% (The following table gives the compositions).

Mixture prepared by dry grinding on a pendulum grinder and wetting to 5% moisture.

Forming and dry pressing with hydraulic presses at a specific pressure of 220 kp / on? with five-way nozzle.

Direct placement in adjacent rows of 8 plates on the 20 cm side in a single-layer rolling furnace with the following characteristics: drying length: 13820 mm preheating and firing: 24747 mm chamber width: 1465 mm maximum firing temp .: 1060 ° C cycle time: approx. 25 min.

Fig. 3 shows the firing curve with the temperature of the oven vapor on the ordinate and the time on the abscissa, which is proportionally correlated with the length measurements of the oven in relation to the transport speed of the plates therein.

Glazing: - Glaze composed of a mixture of three fritters (composition is given in Table 1).

Fry 1: 70% Fry 2: 20% Fry 3: 10% Particle size between 0.2 and 1 mm Glazing temperature = 950 ° C Glazing with roll spreader Length of glazing zone: 1000 mm Application of 30 g glaze per plate Duration: 30 seconds. 467 096 14 When the plates have been transported through the glazing zone, they are inserted, still at a temperature of about 900 ° C, into the second oven part 17, which had the following characteristics: Length: 23311 mm f Width: 1465 mm.

I) This part is kept in the entrance zone at 1000 ° C and the plates are thereby moved in about 4 minutes, which is sufficient for complete curing of the glaze.

After the mentioned zone there is the direct and indirect cooling zone for the plates.

The total length of the machine is thus 62878 mm, with a total plate treatment cycle of 40 minutes.

Example 2 Preparation of floor tiles with the following characteristics: Size: 25 x 25 cm Thickness: 8 mm Porosity: 3% Semi-matt, light beige glaze Characteristics of the mixture: Mixture of clays and feldspar in the following proportions: Feldspar: 45% Clay 1: 30% Clay 2: 25% (Compositions are given in Table 1).

The mixture is prepared by wet grinding with 35% water and sputtering. A Sprayed moisture: 5% Forming by dry pressing with hydraulic presses at a specific pressure of 300 kp / on? and three-way nozzle. 467 096 Direct placement in five rows on a single-layer rolling furnace with the following characteristics: Drying length: 14548 mm Preheating and firing: 32102 mm Roll diameter: 32 cm for the last 12700 mm Width: 1465 mm Maximum temperature: 1150 ° C Cycling time 30 minutes.

The firing curve for the temperature of the flue gases will generally be similar to the course of the curve in Fig. 3, with other values of the ordinate and the abscissa.

Glazing Glaze composed of a single frit, with a particle size between 0.2 and 1 mm.

Glazing temperature = 1050 ° C The composition of the frit is that given in Table 1 (frit 4).

Glazing with roller spreader Length of the glazing zone: 1500 mm Application of 45 g glaze per plate Duration: 30 seconds.

After passing through the glazing zone, the plates are inserted, still at a temperature of about 1000 ° C, into the second part of the oven, which has the following characteristics: length: 22628 mm chamber width: 1465 mm Temperature: 1100 ° C.

The plates pass through the first zone of this oven section, which is maintained at 1100 ° C, for about 4 minutes, which is a sufficient time for curing of the glaze, and then pass through the direct and indirect cooling zone.

The total length of the machine is thus 76695 mm, with a total cycle time of 50 minutes. 467 096 16 Table 1 chemical compositions Majolical clay Grës clay Feldspar sand Fältspat Clay 1 Clay 2 Fritta 1 Fritta 2 Frltta 3 Fritta 4 S102 55.3 61.6 81.0 76.4 73.5 64.8 36.5 50, 2 43.3 65.0 A1203 14.8 20.3 9.4 13.4 22.5 28.5 5.0 5.0 5.5 5.6 Fe2O3 6.0 8.0 0.9 0, 7 1.1 1.8 0.2 0.1 TLO2 0.8 1.1 1.1 1.7 1.7 0.1 9.5 5.0 Cao 14.7 0.6 2.8 1, 0 0.3 0 3 4.4, 7 5 'I MQO 3.2 2.3 0.3 0.3 0.3 0.2 0.3 1.5 3.9 0.1 KZO 3.1 3.8 3 , 4,, 2.1 0.6, 0.8 Na2O 1.2 1.6 2.1,, 1 0.2 0.7,, 4.0 ZHO 4.5 3.2 2.0 BaO, - | C00 10.0 Pb0 34.6 8.8 ZrO2 5.4 5203 5.0 10.4 25.3 10.1 Li2O 0.3 L.O.I. 14.6 6.0 CaCO3 22.3 0.3 4.1 f) 467 096 17 The process according to the invention thus leads to the following advantages over known processes: exclusion of all movement of raw tiles, in addition to their placement in the oven, thus avoiding breakage and flaking; simplification of the manufacturing process, with, if desired, the exclusion of all constructions normally required for the storage of raw plates, provided that the furnace can be fed continuously and directly from the forming section; reducing energy consumption for drying by not partially absorbing glaze water from the raw tiles; additional freedom in the design of marks and embossing on the top and back of the tiles in that there are no problems with differentiated drying, which arise due to different thicknesses or different porosities near the back reliefs; the whole oven, which is fed with unglazed plates, remains clean and thus requires less maintenance; the ceramic body, which is fired without glaze, can emit gas freely and can therefore be more easily oxidized; the burning process benefits and can be performed in a shorter time; you can use less pure clays with higher burning losses; use of engober is completely unnecessary; the firing temperature of the ceramic body is not related to that of the glaze, which in any case may vary.

The glaze is therefore fired under the conditions most suitable for obtaining perfect adhesion and complete absence of porosity by applying it when the reactions in the ceramic body are completed, which makes the use of engobes superfluous; it is possible to apply special glazes which decompose at the temperatures at which the ceramic body is fired, thus widening the range of products which can be produced; the glaze adheres only to the surface of the plate; excess glaze can be easily returned after collection in a funnel; several successive deposits of glaze can be utilized, whereby multiple effects are obtained; since the part of the furnace located downstream of the glazing part is physically separate from the furnace for firing the ceramic body, special atmospheres can be created and regulated therein (oxidizing, reducing or inert atmospheres) without this in any way changes the characteristics of the firing cycle of the mixture. In this way a greater freedom is achieved in performing special effects, which can be obtained in a regulated atmosphere, which is very difficult to achieve using conventional methods; it is possible to produce products with vitrified ceramic bodies much more economically, ie. with high mechanical strength, with completely non-porous glazes, ie. which are inherently resistant to abrasion and have excellent adhesion properties and which therefore also have very high impact strengths; the range of products that can be used for glazing is significantly expanded, making it possible to use low-melting glazes on vitrified ceramic bodies fired at higher temperatures or glazes that require very short firing cycles, which is impossible with conventional procedures.

Claims (3)

A 67 096 20 Patent claim Method for producing a glazed ceramic tile or tile in two layers consisting of ceramic material resp. glaze, without any intermediate layer, starting from a raw, dried, ceramic body, which is placed in a two-stage heat treatment oven with intermediate interruptions in the heat treatment for applying the glaze, characterized in that the plate below it the first heat treatment step is moved on a conveying means through a furnace zone with a sufficiently high temperature and for a sufficient time for the firing reactions to be substantially terminated, after which the heat treatment is stopped and the ceramic body, which is at substantially the final temperature of the first heat treatment step and is in white annealed plastic state, spreads a dry loose glaze, at a temperature for the glaze and within a zone with a temperature lower than the melting temperature of all components of the glaze, the plate, when leaving the first stage, exhibiting a higher temperature than the glaze glazing temperature and the time between the first and the second heat treatment step is so short that the ceramic body maintains a sufficiently high temperature to remain under plastic conditions and not to become brittle, the ceramic plate being maintained at or brought to a temperature during the first part of the second step which is high enough for the glaze to be glazed by melting at least one component part therein. Device for firing and glazing of ceramic tiles, which consists of two layers of ceramic material resp. glaze, without intermediate layers, comprising a kiln or chamber, in which raw, dried ceramic bodies are placed, comprising a first part (11) in the furnace, which comprises means for heating the ceramic raw bodies so that they are burned, a second part (17) in the oven provided with temperature f) 467 096 21 control means for keeping the ceramic bodies at a controlled and decreasing temperature for continuous cooling of the bodies, means for moving the bodies successively through the first and the second part of the oven, and a glazing station (12), characterized in that the oven is provided with a roller transport means (14), which forms a continuous support surface for the plates, which are spaced apart from each other and which are supported by a number of support points, for transporting the plates through the the first part (11), where the plates are heated until the firing reactions are substantially completed, through the glazing zone, which is arranged between the first and the second part of the furnace, between an outlet (13) from the first part (11) and an inlet (16) to the second part (17), and through the second part (17), that the glazing station (12) comprises a spreading device (20), which deposits dry loose glaze on the ceramic bodies and in a collecting device (23) collects the glaze, which falls into the areas between the spaced apart plates and which is located in an environment provided with temperature regulating means, which keep the temperature lower than the temperature at which the glaze undergoes vitrification and physical and chemical changes, the distance between the first part (11) and the second part (17) being small enough that the plates leaving the first part in a white annealed plastic state are not cooled to a fragile state at the passage therethrough. Glazed ceramic tile in two layers consisting of ceramic material resp. glaze, without any intermediate layer, which is prepared from a raw, dried, ceramic body, which is placed in an oven for a heat treatment in two steps with intermediate interruptions in the heat treatment for applying the glaze, characterized in that during the production of the plate, the raw body is moved during the first heat treatment step on a transport means through a zone with a sufficiently high temperature and for a sufficiently long time for the burning reactions to be substantially terminated, after which the heat treatment is stopped and the ceramic the body, which is at substantially the final temperature for the first heat treatment step and is in white annealed. plastic, condition, spreads a dry loose glaze, at a temperature for the glaze and within a zone with a temperature lower than the melting temperature of all components of the glaze, the plate, when leaving the first stage, exhibiting a higher temperature than the glazing temperature of the glaze and the time between the first and second heat treatment steps is so short that the ceramic body maintains a sufficiently high temperature to remain under plastic conditions and not to become brittle, the ceramic plate being kept during the first part of the second step at or brought to a temperature high enough for the glaze to be vitrified by melting at least one component part therein. f)